FieldMatrix should be trivially copyable and standard layout type

Closes: #169

dune/common/fmatrix.hh

@@ -87,7 +87,7 @@ namespace Dune
     //===== constructors
     /** \brief Default constructor
      */
-    FieldMatrix () {}
+    FieldMatrix() = default;
 
     /** \brief Constructor initializing the matrix from a list of vector
      */
@@ -107,16 +107,21 @@ namespace Dune
 
     using Base::operator=;
 
-    // Specialisation: FieldMatrix assignment (compile-time bounds checking)
-    template <typename T, int rows, int cols>
-    FieldMatrix& operator=(FieldMatrix<T,rows,cols> const &rhs)
+    //! copy assignment operator
+    FieldMatrix& operator=(const FieldMatrix&) = default;
+
+    //! copy assignment from FieldMatrix over a different field
+    template<typename T>
+    FieldMatrix& operator=(const FieldMatrix<T, ROWS, COLS>& x)
     {
-      static_assert(rows == ROWS, "Size mismatch in matrix assignment (rows)");
-      static_assert(cols == COLS, "Size mismatch in matrix assignment (columns)");
-      _data = rhs._data;
+      _data = x._data;
       return *this;
     }
 
+    //! no copy assignment from FieldMatrix of different size
+    template <typename T, int rows, int cols>
+    FieldMatrix& operator=(FieldMatrix<T,rows,cols> const&) = delete;
+
     //! Multiplies M from the left to this matrix, this matrix is not modified
     template<int l>
     FieldMatrix<K,l,cols> leftmultiplyany (const FieldMatrix<K,l,rows>& M) const
@@ -226,7 +231,7 @@ namespace Dune
     //===== constructors
     /** \brief Default constructor
      */
-    FieldMatrix () {}
+    FieldMatrix() = default;
 
     /** \brief Constructor initializing the matrix from a list of vector
      */

dune/common/test/fmatrixtest.cc

@@ -733,6 +733,15 @@ void test_interface()
   typedef CheckMatrixInterface::UseFieldVector< K2, rows, cols > Traits;
   typedef Dune::FieldMatrix< K, rows, cols > FMatrix;
 
+  static_assert(
+    !std::is_trivially_copyable<K>::value || std::is_trivially_copyable<FMatrix>::value,
+    "FieldMatrix<T, ...> must be trivally copyable type when T is trivial type"
+    );
+  static_assert(
+    std::is_standard_layout<FMatrix>::value,
+    "FieldMatrix<...> must be a standard layout type"
+    );
+
   FMatrix m( 1 );
   checkMatrixInterface< FMatrix >( m );
   checkMatrixInterface< FMatrix, Traits >( m );